DESCRIPTION (Applicant's Abstract): Brain serotonin (5-HT) levels are dependent on plasma levels of the essential amino acid, tryptophan (TRP). Acute depletion of plasma TRP causes a 6- to 24-hour-long depressive relapse in most depressed patients treated with selective 5-HT reuptake inhibitors and in 80% of euthymic, drug-free subjects with a prior major depressive episode (MDE), but little mood change in untreated, symptomatic depressed patients or young male healthy subjects. The prior work does not allow determination of whether the selective effects of TRP depletion in antidepressant-treated patients are due to interactions of TRP depletion with antidepressant drugs, or whether subjects who are vulnerable to depression are more responsive when in clinical remission. Based on pilot data, we hypothesize that TRP depletion will cause depressive symptoms in medication-free subjects in remission from a prior major depressive episode (MDE) and that severity of depressive symptoms during TRP depletion is associated with risk for a future MDE. Methods: Patients with a prior MDE who have a documented therapeutic response to antidepressant medication and have been continued on medication for at least 6 months and not more than 24 months and want to discontinue antidepressant medication will be recruited. These subjects will be continued on medication for 4 weeks and then antidepressants will be gradually discontinued over an additional 4 weeks. The first 40 subjects who continue in clinical remission off medication for an additional 4 weeks will receive two 3-day test sessions (TRP depletion and control) in a double-blind, placebo-controlled, crossover fashion. Each session involves 3-days: a baseline day, a depletion day (or control), and a follow-up day. TRP depletion involves administration of a TRP-free, 15-amino acid drink and control testing uses a TRP-supplemented, 16-amino acid drink. Sequence of testing is randomly assigned. Comprehensive evaluation and behavioral ratings (Ham-D and Anxiety Scales, Symptom Checklist and Profile of Mood States) and plasma (for TRP levels) are obtained prior to, during, and after testing. Forty age-, gender- and ethnicity-matched healthy subjects (no personal or family history of any mental disorder) will be tested as above. All subjects are followed by face-to-face visits weekly for the first month after testing, then weekly for 11 additional months. Based on prior studies in similar patients, at least 20 patients will experience a new MDE in the year-long follow-up period and these patients will again receive TRP depletion testing as above, prior to receiving free medication treatment for up to three months. Significance: This study will provide important data on the role of the 5-HT system in modulating depressive symptoms in medication-free subjects. Additionally, we will obtain data on the sensitivity, specificity, and positive and negative predictive value of TRP depletion individuals a risk for developing further MDEs. TRP depletion may be a clinically useful tool for identifying people at risk for depression analogous to the way in which a cardiac stress test identifies individuals at risk for heart disease.